The use of carbon materials as conductive aids in battery electrodes is well-known. Virtually all battery systems using particulate materials can be mixed with carbon-based particulates and/or coatings to improve the conductivity and, by extension, discharge performance of the electrode in which they are mixed. Despite the fact that many battery systems rely on carbon conductors, the nature and use of these conductors varies considerably depending upon the intended use and discharge regime (e.g., anode vs. cathode; primary vs. secondary) and battery chemistry (e.g., lithium-ion, lithium-iron disulfide, zinc-air, zinc-manganese dioxide, etc.).
For example, secondary batteries often rely on carbon electrode structures to facilitate the ionic transport within the carbon material during charge and discharge cycling. Consequently, the combination of specific sources of carbons, as well as their orientation and structure, are dictated by the exigencies of this intended use. U.S. Pat. No. 8,765,302 describes a graphene-enabled vanadium oxide composite composition for use as a lithium cathode (i.e., positive electrode) active material exhibiting unprecedented specific capacity, capacity retention and rate capability characteristics. U.S. Pat. No. 8,691,441 discloses mutually bonded or agglomerated graphene sheets and particles for use in lithium battery cathode active materials.
Carbon structures for anode (i.e., negative electrode) materials in secondary cells are known and specifically optimized for ionic intercalation. U.S. Pat. No. 8,440,352 claims a fine carbon powder agglomerated onto the surface of plate-shaped carbon powder particles, with an amorphous carbon coating overlaid onto the powder, with the resulting carbon structure being incorporated into the negative electrode plate of an intercalating, secondary lithium battery. United States Patent Publication 2015/0194668 describes a composite graphite particle for use as an active material in a nonaqueous secondary battery in which the particle comprises graphite and metallic particle capable of alloying with lithium.
Separately, carbons are conductive powders may be mixed with particulate active materials to facilitate conduction of electronics in the electrode structure during discharge. For example, in lithium-iron disulfide primary batteries, U.S. Pat. No. 8,785,044 discloses a cathode formulation relying on a combination of graphite and acetylene black to enable significantly larger amounts of active iron disulfide in the cathode coating. Of course, the electrochemical reaction inherent to this system results in the formation of conductive iron, so the ultimate considerations in the amount and dispersion of carbon in the initial cathode formulation is distinct from other battery systems which produce different reaction products.
As a separate example, U.S. Pat. No. 6,828,064 discloses the use of expanded graphite particles in electrochemical cells, particularly alkaline cells having cathodes formed from a mixture of manganese dioxide and conductive carbon materials forming a conductive matrix. Because manganese dioxide has a relatively low level of conductivity, conductive agents such as graphite, expanded graphite and/or acetylene black are commonly used as conductive agents, although their use entails volumetric and unwanted absorption concerns.
U.S. Pat. No. 8,298,706 provides a generic listing of the range of potential conductive additives in alkaline batteries. Anywhere between 2-35 wt. % of conductive additive can be selected from a list that includes graphite, carbon black, acetylene black, partially graphitized carbon black, carbon fibers and/or nanofibers, carbon nanotubes, and graphene, as well as various non-carbon based conductors (e.g., silver, gold, or nickel powders). The graphite is further characterized as non-synthetic/natural non-expanded graphite, synthetic non-expanded graphite, non-synthetic/natural expanded graphite, synthetic expanded graphite and oxidation-resistant, synthetic non-expanded graphite.
U.S. Pat. No. 8,920,969 also identifies a number of different carbon-based conductor materials that may be appropriate for use in alkaline batteries. However, it is suggested that relatively low weight percentages (less than 3.75 wt. %, with preferred ranges between 2.0 and 3.5 wt. %) of conductor enable the inclusion of higher levels of active material. In the same manner, it is suggested that the conductor consist only of expanded graphite, as unexpanded graphite is significantly less conductive and, by implication, less desirable for use in alkaline or other batteries.
United States Patent Publication Nos. 2012/005229 and 2008/0116423 generally describe agglomerated cathode active material structures for secondary (i.e., rechargeable) nonaqueous batteries. The use of agglomerates in alkaline battery anodes has also been noted in U.S. Pat. Nos. 7,709,144 and 7,332,247.
Mixtures of active material (e.g., manganese dioxide) and conductive carbons usually result in randomized dispersion of the materials. When the conductive carbons have a particular shape (e.g., graphite), the orientation of those shapes will also occur randomly. FIG. 1 illustrates a non-optimized, random distribution of plate-shaped graphite particles dispersed within a manganese dioxide electrode mixture, with manganese dioxide particles or agglomerates M mixed with graphite particles G having essentially random orientations in comparison to the putative current flowing through the mixture (as generally represented by I).
The following description and the drawings disclose various illustrative aspects. Some improvements and novel aspects may be expressly identified, while others may be apparent from the description and drawings.